IV) Characterization and Calibration

The quantities and their uncertainties that dominate the beginning of mission absolute scale for NISTAR are as follows: responsivity of the receiver cavities, aperture area, filter transmission, and temperature dependence of the electronics. This is a partial list of the parameters that will be determined during the calibration and characterization of NISTAR at NIST.

A number of on-orbit checks will be routinely performed to check the stability and degradation of the instrument with time:

1. routine look at deep space to check the 'zero' of the instrument,
2. measurements of solar reflected radiation from the Moon,
3. intercomparisons with redundant filters to track the filter degradation,
4. cross checks between the different channels to look at relative drifts.

A calibration chamber has been developed that will allow complete operational testing of the NISTAR instrument before integration to the Triana spacecraft. The chamber is equipped with a liquid nitrogen shroud, temperature controlled interface/support plate, and four Brewster windows. This will allow the instrument to be temperature cycled throughout its operational range in a low-background environment. An important part of the characterization effort is to determine the end-to-end operational parameters for the flight electronics as a function temperature. The four Brewster windows will allow optical access to the radiometer channels using a polarized laser beam. A complete optical setup including a laser power stabilizer and an optical-trap detector standard has also been developed. The arrangement produces a collimated light source with known power (0.03%) that is stable to 10 ppm/hour. This will allow optical power measurements that are directly traceable to the national standards.